This invention relates generally to optical communications, and more specifically to a system and method for stabilizing the wavelength of a laser or light source.
Integrated optical devices for monitoring and controlling optical signals have become more important as optical fiber communications channels increasingly replace metal cable and microwave transmission links. For example, in many applications, a light source needs to be tuned to a particular wavelength and maintained at that wavelength for an extended period of operation. Unfortunately, it has been recognized that over long periods of usage, laser light sources gradually experience wavelength drift. Furthermore, variations in the operating temperature of a light source may also cause the output wavelength of the light source to fluctuate.
Therefore, several methods have been devised for monitoring and controlling the wavelength of light sources. One of the most common solutions utilizes an error feedback system to monitor and control the output wavelength of the light. For example, a typical control system may use an interferometer, that receives a transmitted optical beam and outputs a filtered optical signal having an amplitude that varies with the wavelength of the transmitted optical beam. The filtered optical signal may therefore be used to stabilize the wavelength of the transmitted opticalbeam.
More specifically, because the amplitude of the filtered optical signal is a function of the amplitude and wavelength of the transmitted optical signal, the ratio of the amplitude of the filtered optical signal to the amplitude of the transmitted optical beam depends on the wavelength of the transmitted optical beam. Therefore, the wavelength of the transmitted optical signal may generally be determined by monitoring the ratio of the filtered optical signal and the transmitted optical beam.
Fabry-Perot etalons are commonly used to provide the wavelength dependent filtered optical signal. An etalon is a type of interference filter in which the intensity of the light transmitted through the etalon is dependent on its wavelength. In a conventional design, an etalon is comprised of two partially reflective parallel surfaces a distance (d) apart and separated by a material with an index of refraction (n). In operation, when collimated light having a wavelength xcex is passed through the etalon, some of the light is reflected from the partially reflective parallel surfaces. The reflected light beams interfere, either constructively or destructively, with each other, and thus alter the overall intensity of the light passing through the etalon.
In operation, maximum transmission through the etalon occurs when the two way optical path (i.e. 2*nd) between the reflective surfaces is an integral number of wavelengths xcex in the etalon, (i.e.2d*n/xcex=x, where x is an integer). Thus, referring to FIG. 1, for a given optical path length, the response curve (i.e. transmission intensity versus wavelength) is periodic having maximum that occur at a spacing given by xcex94xcex=xcex2/2 (nd).
FIG. 1 also shows how the reflectivity of the parallel surfaces affects the transmission. If the reflectivity of the parallel surfaces is relatively high, the maxima of the intensity response will be relatively narrow and sharp. In addition, the intensity response 4 may further include relatively low slope regions between the maxima wherein the intensity does not significantly vary with wavelength. Therefore, the ratio of the amplitude of the filtered optical signal to the transmitted optical beam, also will not vary much in these low slope region, making it difficult to accurately detect and control the output wavelength of the laser in these low slope regions.
In practice therefore, the reflectivity of the parallel surfaces is often reduced to optimize the intensity profile 2 for a particular wavelength stabilization system. However, as seen in FIG. 1, reducing the reflectivity of the parallel surfaces reduces the modulation depth, i.e. separation between minima and maxima, of the device. The reduced modulation depth decreases the systems ability to differentiate small changes in the frequency of the input light.
Therefore, it would be advantageous to provide an interferometer having an intensity response with improved modulation and increased usable bandwidth.
In one aspect of the present invention a monitoring and control apparatus for an optical system includes an interferometer that splits an input beam into a transmitted portion and a reflected portion, wherein the interferometer introduces a path length difference between at least a portion of the transmitted and reflected portions that recombine to form an output beam having an intensity that varies as a function of the wavelength of the input beam.
In another aspect of the present invention a monitoring and control apparatus for an optical system includes an interferometer having a first right angle prism with a first thickness optically coupled along an interface to a second right angle prism with a second thickness, wherein an input beam is incident upon the interface and split into a transmitted portion and a reflected portion, and wherein differences in the first and second thicknesses introduce a path length difference between at least a portion of the transmitted and reflected portions that recombine to form an output beam having an intensity that varies as a function of wavelength of the input beam.
In a further aspect of the present invention, a monitoring and control apparatus for an optical system includes an interferometer having a body with first and second diffraction gratings coupled to parallel opposing sides of the body, wherein the interferometer introduces a pathlength difference for at least a portion of positive and negative diffraction rays that recombine to form an output beam having an intensity that varies as a function of wavelength of the input beam.